A known method of producing polycrystalline silicon suitable for manufacturing solar photocells consists firstly in purifying silicon of metallurgical quality. To do this, the metallurgical silicon is treated with anhydrous hydrochloric acid at a temperature of about 250.degree. C. to form gaseous trichlorosilane which is successively subjected to fractional distillation and to reduction by hydrogen at a temperature of about 1000.degree. C. The method then consists in melting the electronic quality silicon thus obtained in a crucible and in subjecting the mass of molten silicon in the crucible to a vertical temperature gradient, the upper surface of the molten mass being subjected to a temperature slightly lower than the melting point of silicon, e.g. 1400.degree. C. and the lower surface being in contact with the crucible whose temperature is maintained at the most at 1200.degree. C.
The known method described hereinabove has the disadvantage of being very expensive. Indeed, fractional distillation must include a great number of successive distilling operations and the reducing operation is also long and tricky. Further, the device for using the method is very complex and requires considerable investment.
The present invention aims to mitigate this disadvantage and to develop an economical method which makes it possible to reduce the cost price of polycrystalline silicon such as that used for making solar photocells.